1. Field of the Invention
The present invention relates in general to power steering devices for wheeled motor vehicles, and more particularly to the power steering devices of a type which provides a driver with a comfortable steering feeling.
2. Description of the Related Art
In order to clarify the task of the present invention, two power steering devices hitherto proposed will be briefly described with reference to corresponding drawings.
FIG. 9 shows a power steering device disclosed in Laid-Open Japanese Patent Application (Tokkai-sho) 59-118569. The device of this publication generally comprises a steering shaft 2 having a steering wheel 1 connected thereto. An output shaft 3 is connected to steering shaft 2 through a torsion spring (not shown). Output shaft 3 has a pinion 4b connected to a lower end thereof. Pinion 4b is meshed with a rack 4a provided at one end of a piston rod 5. Piston rod 5 has a piston 10 slidably received in a power cylinder 6. Due to provision of piston 10, the interior of power cylinder 6 is divided into first and second work chambers 11a and 11b, as shown. Denoted by numeral 8 is a reversible hydraulic pump driven by an electric motor 7. Pump 8 has two outlet/inlet ports 9a and 9b which are fluidly connected to first and second work chambers 11a and 11b through first and second fluid passages 12a and 12b, respectively. First and second fluid passages 12a and 12b have a bypass passage 15 therebetween. Bypass passage 15 has an electromagnetic switch valve 16 mounted thereon. Denoted by numeral 13 is a torque sensor which detects a steering torque produced when steering shaft 2 is turned. Denoted by numeral 14 is an electronic controller which, based on an information signal from torque sensor 13, controls electric motor 7 and electromagnetic switch valve 16. That is, by controller 14, a rotation direction and rotation speed of electric motor 7 are controlled, so that an actual outlet port 9a or 9b of pump 8 is selected and a delivery pressure by actual outlet port 9a or 9b is controlled. When the detected steering torque is smaller than a predetermined value, controller 14 controls switch valve 16 to open bypass passage 15, while when the torque is larger than the predetermined valve, controller 14 controls valve 16 to close bypass passage 15. Thus, when, under cruising of an associated motor vehicle, steering wheel 1 is turned by a driver and thus torque sensor 13 detects the steering torque, controller 14 controls electric motor 7 and switch valve 16 in the above-mentioned manner. With this, first and second work chambers 11a and 11b of power cylinder 6 are fed with controlled fluid pressure to shift piston 10 in a certain direction, so that the steering operation by the driver is assisted.
Another power steering device is disclosed in Laid-Open Japanese Patent Application (Tokkai-sho) 57-201767. The device of this publication is similar in construction to that of the above-mentioned publication. However, in the device of 57-201767 publication, in place of the above-mentioned electromagnetic switch valve 16, a hydraulically actuated switch valve is employed which is disposed on the bypass passage. Furthermore, respective pilot passages extend from the two outlet/inlet ports of the reversible hydraulic pump to the switch valve. In operation, the switch valve opens and closes the bypass passage in accordance with a magnitude of pilot pressures produced in the pilot passages. When, under cruising of an associated motor vehicle at a middle or high speed, the torque sensor detects substantially no steering torque, the controller stops the motor of the hydraulic pump causing the bypass passage to open. Under this condition, the two work chambers of the power cylinder are communicated with each other, so that steering of the vehicle is carried out without aid of the power of the power cylinder.